The present invention relates to the reproduction of electronically processed mixed (text/continuous tone) images and, more particularly, to the conversion of a scanned multi-bit image into a readily compressible binary image capable of reproduction with improved quality in both the text and continuous tone regions.
The reproduction of mixed images using electronic scanning and processing, such as in facsimile systems, poses a particular problem since known processes that work well on graphics or text areas, having sharp black and white edges in the original images, do not work well on continuous tone or gray scale areas, and vice versa. One form of known process for such reproducing involves converting a scanned multi-bit image to a binary image suitable for electronic transmission. An example of such a system is disclosed in U.S. Pat. No. 4,314,281 to Wiggins et al wherein separate modes are used for processing the text areas and the continuous tone areas of an image, the desired mode being manually selected. Typically, in the former mode, continuous tone areas tend to be degraded in reproduction, while in the latter mode text edge quality is sacrificed. The scanned picture elements or pixels are processed one at a time in this system and conversion is accomplished by comparing each multi-bit pixel to one of 16 thresholds stored in a RAM and presented periodically. Compression of the resulting binary images for transmission is problematical.
An example of a known system for converting a multi-bit image to a binary image specifically designed for compression is found in U.S. Pat. No. 4,084,196 to Tisue et al, wherein in one mode the image is divided into non-overlapping blocks containing 3.times.3 pixels. A quantized level for each block is mapped onto a fixed pattern and the value of each block can be recovered and mapped onto another pattern more appropriate for the output device, which may be a printer. Again, however, different modes are used for text and continuous tone images and one or the other mode is used for processing, resulting in some quality loss. Also, noticeable contours may result in the areas of the reproduction image where the level changes slowly as only the pixels within each block are looked at when producing the output.
The present invention is directed to improving upon these known processes by providing a method and means that deal with both the text and continuous tone areas of an image to be reproduced in the same mode and which produce binary image data in a form that is readily compressible for transmission. Further, the resulting images are of better quality due to a diffused error correction technique.